MODEL ANSWERS TO THE FIFTH HOMEWORK

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1 MODEL ANSWERS TO THE FIFTH HOMEWORK 1. Chapter 3, Section 5: 1 (a) Yes. Given a and b Z, φ(ab) = [ab] = [a][b] = φ(a)φ(b). This map is clearly surjective but not injective. Indeed the kernel is easily seen to be nz. (b) No. Suppose that G is not abelian and that xy yx. Then x 1 y 1 y 1 x 1. On the other hand φ(xy) = (xy) 1 = y 1 x 1 x 1 y 1 = φ(x)φ(y), and one wrong certainly does not make a right. (c) Yes. Suppose that x and y are in G. As G is abelian φ(xy) = (xy) 1 = y 1 x 1 = x 1 y 1 = φ(x)φ(y). Thus φ is a homomorphism. Suppose that a G. Then a is the inverse of b = a 1, so that φ(b) = a. Thus φ is surjective. Suppose that a is in the kernel of φ. Then a 1 = e and so a = e. Thus the kernel of φ is trivial and φ is injective. (d) Yes. φ is a homomorphism as the product of two positive numbers is positive, the product of two negative numbers is positive and the product of a negative and a positive number is negative. This map is clearly surjective. The kernel consists of all positive real numbers. Thus φ is far from injective. (e) Yes. Suppose that x and y are in G. Then φ(xy) = (xy) n = x n y n = φ(x)φ(y). 1

2 In general this map is neither injective nor surjective. For example, if G = Z and n = 2 then the image of φ is 2Z, and for example 1 is not in the image. Now suppose that G = Z 4 and n = 2. Then 2[2] = [4] = [0], so that [2] is in the kernel. 10. We need to check that aha 1 = H for all a G. If we pick a H there is nothing to prove. Now a = f i g j. Conjugation by a is the same as conjugation by g j followed by conjugation by f i. So we only need to worry about conjugation by f. Now gf = fg 1 so that fgf 1 = g 1. Thus conjugation by f leaves H fixed, as it sends a generator to a generator. 12. Let g G. We want to show that gzg 1 Z. Pick z Z. Then z commutes with g, so that gzg 1 = zgg 1 = z Z. Thus Z is normal in G. 17. Let g G. We want to show that g(m N)g 1 M N. Pick h M N. Then h M and h N. It follows that ghg 1 M and ghg 1 N, as both M and N are normal in G. But then ghg 1 M N and so M N is normal. 22. H = {i, (1, 2)}. Then the left cosets of H are (a) and the right cosets are (b) H = {e, (1, 2)} (1, 3)H = {(1, 3), (1, 2, 3)} (2, 3)H = {(2, 3), (1, 3, 2)} H = {e, (1, 2)} H(1, 3) = {(1, 3), (1, 3, 2)} H(2, 3) = {(2, 3), (1, 2, 3)}. (c) Clearly not every left coset is a right coset. For example {(1, 3), (1, 2, 3)} is a left coset, but not a right coset. 27. Let g G. We have to show that gθ(n)g 1 θ(n). Now as θ is surjective, we may write g = θ(h), for some h G. Pick m θ(n). Then m = θ(n), for some n N. We have gmg 1 = θ(h)θ(n)θ(h) 1 = θ(hnh 1 ). 2

3 Now hnh 1 N as N is normal. So gmg 1 θ(n) and θ(n) is normal in G. 37. Note that S 3 is the group of permutations of three objects. So we want to find three things on which G acts. Pick any element h of G. Then the order of h divides the order of G. As the order of G is six, it follows that the order of h is one, two, three, or six. It cannot be six, as then G would be cyclic, whence abelian, and it can only be one if h is the identity. Note that elements of order 3 come in pairs. If a is an element of order 3 then a 2 = a 1 also has order three and they are the two elements of a not equal to the identity. So the number of elements of order 3 is even. As there are five elements of G which don t have order one, it follows that at least one element a of G has order 2. If H = a then H is a subgroup of G of order two. Let b be any other element of G. Consider the subgroup K = a, b of G generated by a and b. Then K has at least three elements, e, a and b and on the other hand the order of K is even by Lagrange as H is a subgroup of order 2. Thus K has at least four elements. As the order of K divides the order of G the order of K is six, so that G = a, b is generated by a and b. If ab = ba it is not hard to check that G is abelian. As G is not abelian we must have ab ba. As H is a subgroup of G of order two, the number of left cosets of H in G (the index of H in G) is equal to three, by Lagrange. Let S be the set of left cosets. Define a map from G to A(S), φ: G A(S) by sending g to σ = φ(g), where σ is the map, σ : S S σ(xh) = gxh, that is, σ acts on the left cosets by left multiplication by g. If xh = yh so that y = xh for some h H then gy = g(xh) = (gx)h, so that (gy)h = (gx)h and σ is well-defined. σ is a bijection, as its inverse τ is given by left multiplication by g 1. Now we check that φ is a homomorphism. Suppose that g 1 and g 2 are two elements of G. Set σ i = φ(g i ) and let τ = φ(g 1 g 2 ). We need to check that τ = σ 1 σ 2. Pick 3

4 a left coset xh. Then σ 1 σ 2 (xh) = σ 1 (g 2 xh) = g 1 g 2 xh = τ(xh). Thus φ is a homomorphism. We check that φ is injective. It suffices to prove that the kernel of φ is trivial. Pick g Ker φ. Then σ = φ(g) is the identity permutation, so that for every left coset xh, gxh = xh. Consider the left coset H. Then gh = H. It follows that g H, so that either g = e or g = a. If g = a, then consider the left coset bh. We would then have abh = bh, so that ab = bh, where h H. So h = e or h = a. If h = e, then ab = b, and a = e, a contradiction. Otherwise ab = ba, a contradiction. Thus g = e, the kernel of φ is trivial and φ is injective. As A(S) has order six and φ is injective, it follows that φ is a bijection. Thus G is isomorphic to S Let G be a group of order nine. Let g G be an element of G. Then the order of g divides the order of G. Thus the order of g is 1, 3 or 9. If G is cyclic then G is certainly abelian. Thus we may assume that there is no element of order nine. On the other hand the order of g is one if and only if g = e. Thus we may assume that every element of G, other than the identity, has order three. Let a G be an element of G, other than the identity. Let H = a. Then H has order three. Let S be the set of left cosets of H in G. By Lagrange S has three elements. Let φ: G A(S) S 3 be the corresponding homomorphism. Let G be the order of the image. Then the order of G is the number of left cosets of the kernel, which divides G by Lagrange. On the other hand the order of G divides the order of A(S), again by Lagrange. Thus G must have order three. It follows that the kernel of φ has order three. Thus the kernel of φ is H and H is a normal subgroup of G. Let b G be any element of G that does not commute with a. Then bab 1 must be an element of H, as H is normal in G, and so bab 1 = a 2. 4

5 It follows that ba = a 2 b 2. In this case (ba) 2 = baba = a 2 b 2 ba = a 2 a = e. Thus ba is an element of order 2, which is impossible as G has order Let S be the set of left cosets of H in G. Define a map φ: G A(S) by sending g G to the permutation σ A(S), a map σ : S S defined by the rule σ(ah) = gah. Note that τ, which acts by multiplication on the left by g 1 is the inverse of σ, so that σ is indeed a permutation of S. It is easy to check, as before, that φ is a homomorphism. Let N be the kernel of φ. Then N is normal in G. Suppose that a N and let σ = φ(a). Then σ is the identity permutation of S. In particular σ(h) = H, so that ah = H. Thus a H and so N H. Let n be the index of H, so that the image of G has at most n! elements. In this case there are at most n! left cosets of N in G, since each left coset of N in G is mapped to a different element of A(S). Thus the index of N is at most n!. 52. Let A be the set of elements such that φ(a) = a 1. Pick an element g G and let B = g 1 A. Then A B = A + B A B > (3/4) G + (3/4) G G = (1/2) G. Now pick h A B and suppose that g A. Then gh A. It follows that h 1 g 1 = (gh) 1 = φ(gh) = φ(g)φ(h) = g 1 h 1. Taking inverses, we see that g and h must commute. Let C be the centraliser of g. Then A B C, so that C contains more than half the elements of G. On the other hand, C is subgroup of G. By 5

6 Lagrange the order of C divides the order of G. Thus C = G. Hence g is in the centre Z of G and so the centre Z of G contains at least 3/4 of the elements of G. But then the centre of G must also equal G, as it is also a subgroup of G. Thus G is abelian. 6